In recent years, with rapid expansion in diffusion of network services typified by the Internet and cellular telephones, expectations have been placed on realization of more sophisticated communication services. In the cellular telephones, the so-called handover function and mobility management function have been provided such that communications are continued when a user moves while bringing a terminal. By further developing the functions to be more sophisticated, actualization and the like are desired of functions for enabling communications to be continued when the terminal is changed during the communication so as to maintain the consistent service.
When communication networks have been expanded and developed and various kinds of communication terminals can connect to the communication networks, users have many types of communication terminals connectable to the communication networks. As these communication terminals, not only portable terminals such as cellular telephones, but also stationary apparatuses such as, for example, TV receivers and the like installed in the home are expected to function as communication terminals.
Meanwhile, by providing the common IP backbone using IP (Internet Protocol) communication techniques constructed by development of the Internet on the communication network side, it is possible to receive common services in various kinds of communication terminals without much concern for differences in access means such as wireless, cable and the like, operators of the networks, and service providers.
Under such circumstances, a user uses communication terminals such as, for example, a cellular telephone, PDA, personal computer, TV receiver and the like that the user has separately without restraint corresponding to his/her own status, and such a requirement arises that the user wants to use the same service using a communication terminal that the user has and selects at this point.
Further, such a case is expected that the user switches the communication terminal being used to another communication terminal while continuing the service. In this case, it is necessary to achieve user demand for shifting the communication state to another terminal seamlessly while maintaining the communication state to use the service continuously, without disconnecting communications once and newly connecting from another communication terminal. More specifically, there is an example that a user uses the service for receiving image content in streaming distribution at a personal computer at home, and when going out, switches the reception terminal to a portable communication terminal such as a cellular telephone, PDA or the like to bring away from home so as to use the same service continuously. Further, another example is considered that a user uses video telephone service away from home, gets home while communicating with the communicating apparatus, shifts the service to a TV receiver at home, and continues the video telephone service with the communicating apparatus.
In implementing seamless switching between communication terminals as described above, when the communication service is multi-cast type distribution and the like, since data of the same content is simultaneously distributed from a communication terminal on the distribution side to a plurality of (an indefinite number of) communication terminals, the user is only required to connect the switched reception terminal to the terminal on the distribution side to receive the data in the reception terminal, and is capable of implementing switching between communication terminals relatively with ease.
However, in uni-cast type communication where specific data is distributed to some specific communication terminal or one-to-one communication, the user needs procedures for once instructing to a communication terminal on the distribution side to suspend communications, connecting to the distribution-side terminal from a desired switched terminal, notifying switching of the reception-side terminal, and resuming the communications.
FIG. 6 shows an example of switching between communication terminals in such a conventional communication network control system. In this example, such a case is shown that during communications of a communication terminal #1 with a server apparatus X, the user switches the communication terminal #1 being used in the communications to a communication terminal #2. In this case, first, the user transmits control information shown by arrow 101 to the server apparatus X on the data distribution side from the communication terminal #1 receiving the data, and instructs to suspend distribution. Next, the user transmits control information shown by arrow 102 for requesting disconnection from the server apparatus X to a control apparatus Y on the communication network, and disconnects the communications.
Then, the user designates address information of the same server apparatus X as in the foregoing as shown by arrow 103 to the control apparatus Y of the communication network from the communication terminal #2 of a switching destination, and transmits a connection request. The control apparatus Y sets a communication path between the communication terminal #2 and server apparatus X on the communication network, and further sets connection between the apparatus X and terminal #2. Subsequently, when the communication terminal #2 transmits control information shown by arrow 104 for instructing to resume the distribution to the server apparatus X, the server apparatus X transmits the data of from the time the distribution is suspended, and the user is thereby capable of receiving subsequent data using the communication terminal #2 and thus implementing switching between terminals to use the service.
Herein, the address information of the server apparatus X for the communication terminal #2 of the switching destination to connect to the server apparatus X is designated with means for the user to operate the communication terminal #2 to input and the like in a form of a telephone number, URL or the like. In this example, the control apparatus Y of the communication network only performs connection and disconnection between the communication terminal #1 or #2 and the server apparatus X, and does not have the control function to switch between terminals, and switching between terminals is achieved by functions of suspending and resuming communications in the communication terminals and server apparatus.
FIG. 7 shows a conventional example of another method of switching between communication terminals. This example also shows the case during communications of the communication terminal #1 with the server apparatus X, the user switches the communication terminal #1 being used in the communications to the communication terminal #2. In switching, the communication terminal #1 on the reception side transmits control information to instruct to suspend distribution to the server X on the distribution side as shown by arrow 201. After suspending communications with the server X, when user performs transfer operation, the communication terminal #1 transmits control information for suspending connection to the server apparatus X, establishing connection of the communication terminal #2, and requesting transfer of the communications to the control apparatus Y of the communication network as shown by arrow 202.
The control apparatus Y receiving the control information transmits control information for instructing to connect to the server apparatus X to the communication terminal #2 as shown by arrow 203 and thereby places a call to the communication terminal #2, and when the communication terminal #2 responds to the call as shown by arrow 204, the communication channel is set between communication terminal #2 and server apparatus X to establish connection.
Subsequently, by transmission and reception of the control information between the communication terminal #1 of a transfer source and control apparatus Y as shown by arrow 205, the connection between the communication terminal #1 and server apparatus X is disconnected, and the communication terminal #2 transmits control information shown by arrow 206 for requesting to resume suspended communications to the server apparatus X, and thus receives subsequent data.
In this example, the control apparatus Y on the communication network has the control function of setting call connection to another terminal to transfer, but does not have the function of controlling suspension and resuming of data transmission/reception between the communication terminal and server apparatus, not guaranteeing continuation of communications i.e. seamless switching.
Further, in this example, the control apparatus Y on the communication network only transfers call connection from one communication terminal to another communication terminal, while neither considering the speech reproduction capability and display capability of the communication terminal #2 of the switching destination nor converting to a content form suitable for the capability, and therefore, the user needs to request conversion to content data suitable for the terminal capability to the server apparatus X from the communication terminal #2 of the transfer destination.    Non-patent Document 1: Yabusaki et al. “Story on Mobile Network All-IP” NTT DoCoMo Technical Journal Vol. 10, No. 4 January, 2003, P. 6-34    Non-patent Document 2: Imai et al. “Service handoff mechanism to achieve ubiquitous applications”, 4th CINAG symposium, 2002, P. 112-113    Non-patent Document 3: Kaneko et al. “Session layer mobility support for dynamic network service”, 4th CINAG symposium, 2002, P. 114-115